5.2.6 Octahedral & Tetrahedral Complexes

• Octahedral complexes are formed when a central metal atom or ion forms six coordinate bonds
• This could be six coordinate bonds with six small, monodentate ligands
  • Examples of such ligands are water and ammonia molecules and hydroxide and thiocyanate ions
  • As there are six ligands, these complexes are sometimes described as having six-fold coordination

Table Showing Examples of Common Monodentate Ligands

Example of an octahedral complex with monodentate ligands

• It could be six-coordinate bonds with three bidentate ligands
  
  • Each bidentate ligand will form two coordinate bonds, meaning six-coordinate bonds in total
  • Examples of these ligands are 1,2-diaminoethane and the ethanedioate ion

Example of an octahedral complex with bidentate ligands

• It could be six-coordinate bonds with one multidentate ligand
  • The multidentate ligand, for example, EDTA^4-_, forms all six-coordinate bonds

Example of an octahedral complex with a polydentate ligand 

• The bond angles in an octahedral complex are 90^o
• The coordination number of a complex is the number of dative bonds formed between the central metal ion and the ligands
  • Since there are 6 dative bonds, the coordination number for the complex is 6

Exam Tip

Tetrahedral complexes

• When there are four coordinate bonds the complexes often have a tetrahedral shape
  • Complexes with four chloride ions most commonly adopt this geometry
  • Chloride ligands are large, so only four will fit around the central metal ion
• The bond angles in tetrahedral complexes are 109.5^o

Example of a tetrahedral complex